An established clinical characteristic variable is the Fractional Flow Reserve (FFR), which may be measured with a pressure wire, for example. In such cases, the pressure wire is guided past a stenosis in the body vessel or body vessel segment and determines the pressure there distal to the stenosis. This distal pressure is divided by the proximal pressure in order to calculate the fractional flow reserve.
It is possible, by a three-dimensional model of the body vessel segment or body vessel section in which the stenosis is contained, and further boundary conditions, (e.g., the blood flow in milliliters per second through the body vessel segment), to calculate the pressure curve via the stenosis by mathematical methods of fluid dynamics (e.g., computational fluid dynamics) and to compute a virtual value for the fractional flow reserve, a virtual FFR value, virtually on the basis of the three-dimensional model. Methods of this type are known and are described for example in the article by Paul D. Morris et al., “Virtual” (Computed) Fractional Flow Reserve—Current Challenges and Limitations,” in JACC: Cardiovascular Interventions, Vol. 8, No. 8, 2015, pages 1009 to 1117, or the article by Charles A. Taylor et al., “Computational fluid dynamics applied to cardiac computed tomography for noninvasive quantification of fractional flow reserve,” in: Journal of the American College of Cardiology, Vol. 61, No. 22, 2013, pages 2233 to 2241. Other methods of computation for a virtual FFR value are also known.
The approaches to virtual computation of the fractional flow reserve may be divided up into two groups: (1) non-invasive methods, in which geometry information about the body vessel segment or body vessel is obtained by computed tomography (CT), magnetic resonance tomography, or other methods; and (2) minimally-invasive methods, in which the geometry information is obtained in the cardiac catheter laboratory by an injection of contrast medium into the vessel with a subsequent x-ray recording. As a rule, a non-invasive examination of a patient is initially undertaken by computed tomography. As well as the diagnostic information about one or more vessel cross sections of the examined body vessel segment or body vessel, a virtual value for the fractional flow reserve may also be computed in such cases, which will be referred to below as the CT FFR value. By contrast, a virtual value for a fractional flow reserve, which is established by an angiography in the cardiac catheter laboratory, for example, will be referred to below as an angio FFR value.
The CT FFR method, (e.g., the computation of the virtual FFR value by a CT), has the advantage in this case that a three-dimensional model of the entire vascular tree, in which the body vessel or the body vessel segment with the stenosis is located, is available. It also allows a good determination of the perfused myocardial mass as well as of the perfusion flow derived from the proportion of the perfused myocardial mass. Furthermore additional information, such as a composition of the stenosis or of the plaque, may be established. The disadvantage in this case is the comparatively low spatial resolution and thus an imprecise geometry representation of the stenosis geometry.
By comparison with this method, the angio FFR method, e.g., the computation of a virtual FFR value using an angiography, has the advantage of a good spatial resolution, which makes a precise representation of the stenosis geometry possible. A disadvantage in this case is the estimation of the blood flow via the vessel cross sections. Here, even small errors may have large effects. The estimation of the blood flow via contrast media dynamics is complex and difficult in the angio FFR method. A further disadvantage is that the angio FFR method does not deliver any information about a state of the myocardial mass, which is important, for example, for recognizing any possible prior damage and enabling it to be considered during a treatment. Above and beyond this, geometry information of the vascular tree as a whole may only be obtained with great difficulty, which may be attributable to the relatively small detectors used in angiographies.
Also based on the results of the CT FFR method, the doctor may be presented with a suggested therapy. For this purpose, based on available information such as a computed tomography, a three-dimensional reconstruction derived from the computed tomography or models reduced or configured in another way, one or more different virtual treatments are carried out, and their success or possible influence is assessed on the basis of a newly calculated virtual FFR value or another clinical characteristic variable. At the end of such a process, planning information is then available about the treatment to be carried out, meaning the therapeutic intervention. The planning information in this case relates to the treatment method, for example, to a stent to be implanted, in the form of dimensions of the stent or a model of the stent, and/or a balloon dilatation and/or other measures, as well as to the treatment location in the body vessel or body vessel segment in the form of a precise localization and alignment as well as the value of the clinical characteristic variable, (e.g., the FFR value), to be achieved in accordance with the treatment.